Cathodic protection is a method of protecting underground and underwater metal structures against corrosion. Protection is obtained by causing a DC generator to pass current between anodes placed in the electrolyte (water or ground) and isolated from the structure, and a cathode constituted by the structure to be protected.
The polarization conditions of metals that need protecting vary depending on a large number of factors: variations in the insulating resistance of a protective covering, if any; variations in the resistivity of the electrolyte; the influence of the speed of electrolyte flow; the formation of deposits on the parts to be protected; etc.
The characteristics of the DC generator thus need to be adapted so that the polarization is maintained at a suitable value.
In prior cathodic protection systems, the power level of the generator is often manually adjusted. For example, the voltage at the output from the generator may be varied by means of a variable annular auto-transformer, a magnetically saturateable transformer, a multiply-tapped transformer, or by means of circuits using thyristors or triacs.
The generator power level may also be regulated automatically by servocontrol means. In this case, the servocontrol means compare a reference potential to be obtained with the potential of the structure to be protected as measured by means of a reference electrode permanently placed on said structure, and said means deliver a control signal as a function of the comparison. Said signal is then amplified and transformed into a current which is caused to flow between said anodes and the structure to be protected in such a direction as to ensure that the current tends to return the potential of the structure to the reference value.
Such a system may be referred to as a "potentiostat".
However, two factors limit industrial application of this type of regulation:
firstly the variations in polarization conditions may be very large, in particular when protection is being applied to pumps, turbines, filter grids, and other apparatuses and structures in which the speed or composition (inter alia) of the electrolyte are subject to large variations; this requires the potentiostat to be overdimensioned if it is to work properly; and PA1 secondly, in some structures such as those mentioned above, the shapes of the parts to be protected and the configuration of the electric field due to the protection current require several series of anodes to be used with a single structure, with each series of anodes being fed from a different generator, even though all of the anodes co-operate in an electrolytic circuit which is common to the entire system of anodes and the cathode.
As mentioned above, in the first place the potentiostat needs to be overdimensioned in order to be sure that it operates properly in all polarization conditions.
In the second place, it is necessary to have as many potentiostats as there are series of anodes corresponding to different portions of the machine to be protected. Interactions occur between the various potentiostats due both to the electrical continuity between the various metal parts of the structure to be protected and also to the common electrolytic medium constituted by the moving liquid, which interactions give rise to instabilities in potential and to phenomena of current oscillation at the outputs from the potentiostats.
Preferred embodiments of the present invention remedy the above drawbacks and allow a current generator for providing cathodic protection to adapt automatically to variations in polarization conditions without it being necessary to overdimension the system, and while simultaneously reducing the influence of interference on the potential of the structure to be protected, which interference is due to the presence of an electrolyte which is common to the various anode-to-cathode circuits.
Preferred embodiments of the invention also provide a system which is also applicable to at least two distinct cathodes which are polarized by respective generators and anodes in a configuration such that the electric fields of the anode-rectifier-cathode assemblies interfere.